(Meth)acrylic acid esters of polyhydric alcohols, more particularly from the group of dihydric to tetrahydric aliphatic saturated alcohols and alkoxylation products thereof, are being increasingly used as highly reactive constituents in radiation-curing systems. Polyfunctional (meth)acrylic acid esters may be used, for example, as paint raw materials for electron beam hardening or as a constituent of UV-hardening printing inks or corresponding coatings, surfacing, molding or potting compounds or even in adhesives, particularly anaerobically curing adhesives. However, their production is not without problems. There is a demand above all for colorless products with a low acid value and high storage stability which also have virtually no odor of their own. Purification of the (meth)acrylic acid esters in question here by distillation is generally not possible on account of their high molecular weight and their high reactivity. Accordingly, the products should be directly obtained as colorless esterification products. Carrying out of the esterification reactions involves the presence of highly effective inhibitors which, for their part, do not initiate any unwanted secondary reactions, for example in the form of discolorations. In addition, it can be desirable not only to protect the liquid reaction product from unwanted polymerization reactions during the esterification reaction, but also to ensure adequate inhibition of the entire reaction space including both the inner gas space and the wall surfaces which come into contact with the inner gas space. This counteracts the danger of unwanted polymer formation, for example on unprotected walls, the washing off of such polymers into the reaction product leading to an unwanted increase in the viscosity of the end product or to unwanted insoluble particles.
However, the invention is not confined to the production of (meth)acrylic acid esters, but instead relates to any esters of unsaturated carboxylic acids with a higher boiling point than water and polyhydric alcohols.
The above-described process is known, for example, for DE 3843843 A1. The reaction space is purged with an oxygen-containing gas stream and that part of the inner reaction space which is filled with the gas phase is charged with the fine droplets of liquid which contain the polymerization inhibitor. In this way, not only is the reactive liquid phase effectively stabilized by polymerization inhibitors, the entire inner reaction space is protected against unwanted polymerization reactions. The same inhibitor is used both for protecting the reactive liquid phase and for protecting the gas- or vapor-filled inner space and the solid surfaces (inner walls, stirrer components, etc.) arranged therein. In another process of the type mentioned at the beginning, the water of condensation formed during the reaction is removed from the gas phase of the reaction space. Finally, DE 3843930 A1 describes the polymerization inhibitors preferably used in the process mentioned at the beginning.
In the acid-catalyzed batch or semibatch esterification process, an inhibitor system which acts chemically in the liquid reaction mixture and which consists, for example, of a hydroquinone derivative in combination with atmospheric oxygen is used to prevent polymerization both of the unsaturated carboxylic acid used and of the unsaturated polyol ester formed. During the esterification, the water of reaction formed is removed from the reaction mixture by distillation under reduced pressure to obtain high conversion levels. With volatile unsaturated carboxylic acids, such as acrylic and methacrylic acid, a water/carboxylic acid mixture with a composition corresponding to the phase equilbrium is removed from the circuit. This known process has two major disadvantages when used for the esterification of volatile unsaturated carboxylic acids:                First, the removal of water/carboxylic acid from the circuit inevitably leads to a large stoichiometric excess of carboxylic acid, based on the polyol used, which can amount to between 40 and 50% in the case of tri- and tetrahydric polyols.        Second, the hydroquinone/air system—which only has an inhibiting effect in the liquid reaction mixture—is attended by the danger that the noninhibited unsaturated carboxylic acid present in the gas phase during the removal of water from the circuit polymerizes during the condensation, leading to serious problems in the process.        
Accordingly, the problem addressed by the present invention was significantly to reduce the excess of carboxylic acid in the process mentioned at the beginning and to prevent the polymerization of carboxylic acid or ester in the liquid or gas phase. In addition, the reaction time would be shortened and the level of organic pollutants in the wastewater wold be reduced.